Conventionally, in order for a metal tube to have desired surface characteristics (corrosion resistance, abrasion or erosion resistance, etc.), or to increase the strength of the tube, the outer periphery or inner periphery of the tube is weld overlaid. For example, Patent Literature 1 discloses a device for weld overlaying the outer periphery of the tube in this manner.
The weld overlay system disclosed in Patent Literature 1 includes a tube holder which holds and rotates a tube, a preheat holder which preheats a weld overlay material and holds the weld overlay material adjacently to the tube, a welding robot including a welding torch (GTAW (gas shielded tungsten arc welding) torch) at a tip end thereof, and a travel device which moves the welding robot in the longitudinal direction of the tube. The welding torch is positioned at 20 to 35 degrees from the top of the tube. By use of this weld overlay system, a filler metal and a base metal (tube) are melted by arch heat generated between the welding torch and the base metal while rotating the tube, and thereby a bead of the melted overlay material of a spiral shape is formed on the surface of the base metal. During the welding, the welding torch oscillates in the longitudinal direction of the tube, and the vertical position of the welding torch is controlled to provide an appropriate arc length, by an AVC (arc voltage control) device. In addition, to control the temperature of the base metal during the welding, water is flowed through the tube.